The present invention relates to the preparation of dry, carboxylic acid salt particles and particularly to methods of neutralizing carboxylic acids using alkali metal-containing or ammonium-containing alkaline compounds to produce safe and useful herbicidal carboxylic acid salts.
Carboxylic acids such as benzoic acids, phenoxy carboxylic acids, pyridine carboxylic acids and quinoline carboxylic acids have been found to be useful in controlling the growth of weeds in various crops. In particular, these carboxylic acids are classified as growth regulator herbicides and act as synthetic auxins in preventing the growth of weeds in crops. A herbicidally effective amount of these carboxylic acid herbicides is applied to the locus of the weeds generally with agriculturally acceptable carriers, surfactants and other additives to control the growth of the weeds.
Because the carboxylic acid herbicides are generally insoluble in water, the carboxylic acids have traditionally been converted to their ester or salt forms for application to weed loci. In these forms, the carboxylic acids can be transported as concentrated solutions and then diluted for actual use. For example, the carboxylic acid esters are generally provided as petroleum distillate-based emulsifiable concentrates and the carboxylic acid salts are typically provided as aqueous concentrates, both of which are diluted with water for actual use.
The carboxylic acid salts have become the preferred forms for these herbicides. In particular, the ammonium and alkali metal salts are commonly used to control the growth of broadleaf weeds. In addition to providing these carboxylic acid salts as liquid concentrates, these carboxylic salts can also be provided in granular form.
The conventional method of preparing granular carboxylic acid salts is to mix and react an aqueous carboxylic acid solution and an alkali metal- or ammonium-containing alkaline solution to produce the carboxylic acid salt and water. The carboxylic acid salt is then dried to form either a liquid concentrate or the granular herbicide.
One problem with this conventional method is cost. In particular, to dry the carboxylic acid salt after its formation, the salt must be heated. Because of the volume of water that is used to form the solution and produced during the formation of the carboxylic acid salt, it is expensive to provide the necessary amount of heat energy to dry the carboxylic acid salt solution to form the liquid concentrate or granular herbicide.
Furthermore, there may be complications associated with conventional dryers used to dry carboxylic acid salt solutions. For example, although spray dryers are commonly used to dry these solutions, the amount of water that needs to be added to the solution often must be increased to allow for effective atomization of the feed. Additionally, because extra water is often needed, the inlet gas temperatures required to dry the material economically may exceed the safe operating temperature of the material. As a result, the dryers must be operated to avoid the risk of fire or explosion of the product. The spray dryer is either operated at the higher desired temperature with an inert atmosphere, which is expensive, or the spray dryer is operated at low and safe temperatures, which substantially reduces the production rate.
Another issue related to the production of carboxylic acid salts is the pH of the carboxylic acid salt solution. Various methods have been employed to control the pH of the carboxylic acid salt solution and the resulting carboxylic acid salts. In particular, because the resulting carboxylic acid salts are generally later dissolved in water for their eventual use, it is important to have salts that produce a desirable pH when dissolved in water.
One method of controlling the pH of the carboxylic acid salt solution is to neutralize the solution as needed prior to drying the carboxylic acid salts. For example, buffers such as alkaline compounds have been combined with the carboxylic acid salts to produce the desired pH for the carboxylic acid solution. U.S. Pat. No. 5,266,553 describes a method of producing carboxylic acid salts by forming the carboxylic acid salts from the carboxylic acid and an alkaline compound and then reacting the carboxylic acid salt with a second alkaline buffer compound to produce a carboxylic acid salt having increased water volubility.
An alternative method of controlling the pH of the salts is to prepare different batches of carboxylic acid salts and to mix the batches to provide the desired pH. However, it is difficult to dry process salts, particularly when they are sticky, to produce a consistent product.
Using buffers and blending salt batches are not efficient methods of producing granular carboxylic acid salt herbicides. Therefore, there is a need in the art to provide a method of consistently producing dry, carboxylic acid salts.
The present invention provides a method for consistently producing dry, carboxylic acid salts. In the production of these salts by the present method, the carboxylic acid is substantially fully neutralized. As a result, the carboxylic acid salts of the invention do not require blending or the use of buffers to produce a desired pH. Therefore, the carboxylic acid salts produced according to the invention can be immediately packaged and transported for use. Furthermore, the carboxylic acid salts of the invention can also be produced at a lower cost because of reduced heating costs in their formation.
The benefits of the present invention are provided by a method of preparing an alkali metal or an ammonium carboxylic acid salt by feeding a molten carboxylic acid and an alkali metal-containing or ammonium-containing alkaline compound in solution to a mixer and mixing the carboxylic acid and alkaline compound. Preferably, the alkaline compound is fed to the mixer in an amount within about 1%, and preferably within 0.6%, of the molar amount sufficient to neutralize the carboxylic acid fed to the mixer. Preferably, the molten carboxylic acid is also substantially free of water. The molten carboxylic acid and the alkali metal-containing or ammonium-containing alkaline compound react to form an alkali metal or ammonium carboxylic acid salt and the carboxylic acid is substantially neutralized, i.e., less than about 1% of the carboxylic acid fed to the process remains after the reacting step. At least a portion of the reacting step and typically substantially all of the reacting step occurs in the mixer. The carboxylic acid salt and any remaining carboxylic acid and alkaline compound are advanced from the mixer to a dryer where water (formed by the reaction and fed with the alkaline compound) is removed from the alkali metal or ammonium carboxylic acid salt to produce dry, alkali metal or ammonium carboxylic acid salt particles. The dry, alkali metal or ammonium carboxylic acid salt particles are then recovered as product and do not require buffers or batch mixing to adjust the pH of the particles. The method steps of the invention typically form a portion of a continuous process for making carboxylic acid salts.
In accordance with the invention, the carboxylic acids used in the method of preparing carboxylic acid salts are preferably growth regulating herbicides selected from the group consisting of benzoic acid herbicides, phenoxy carboxylic acid herbicides, pyridine carboxylic acid herbicides and quinoline carboxylic acid herbicides. 2-Methoxy-3,6-dichlorobenzoic acid (dicamba) is particularly preferred for use in the invention. The alkali metal-containing or ammonium-containing alkaline compound is preferably a sodium compound such as sodium hydroxide or sodium bicarbonate.
In one preferred embodiment of the invention, the molten carboxylic acid and the solution of the alkali metal-containing or ammonium-containing alkaline compound are metered to the mixer. In accordance with the invention, it is preferred that the ratio of the feed components are maintained such that the amount of alkaline compound metered to the mixer or the dryer is consistently within about 1% of the molar amount sufficient to neutralize the carboxylic acid. At least a portion of the reaction and typically substantially all of the reaction occurs in the mixer such that less than about 1% of the carboxylic acid fed to the mixer is advanced to the dryer. The mixer and dryer are preferably both insulated so that any heat of neutralization produced in the reacting step can be used to aid in removing water from the resulting carboxylic acid salt. The dryer is also preferably heated to produce the dry, carboxylic acid salt particles and can be operated under a vacuum, in a nitrogen atmosphere, or both. The dryer is also preferably self-cleaning especially when the resulting carboxylic acid salt solutions strongly adhere to the hot dryer surfaces, i.e., are sticky. The resulting dry, carboxylic acid salt particles include preferably less than 5% and more preferably less than 1% water by weight.
In another preferred embodiment of the invention, sodium dicambate is prepared by mixing molten dicamba with an aqueous sodium hydroxide solution at a molar ratio of 1:0.97+/xe2x88x920.6%, i.e., at a ratio of between 1:0.964 and 1:0.976, and preferably at a substantially constant molar ratio between 1:0.964 to 1:0.976. Preferably, the molten dicamba is substantially free of water. The molten dicamba and the solution of the alkaline sodium compound are preferably fed to a mixer and then advanced to a dryer. The molten dicamba and the alkaline sodium compound are reacted to produce the sodium salt of dicamba (sodium dicambate) and the dicamba is substantially neutralized. Water is then removed from the sodium dicambate to produce dry, sodium dicambate particles having a pH, when dissolved in water, of between about 7 and about 9. The dry, sodium dicambate particles are then recovered as product.
The method of the invention can produce dry, carboxylic acid salts without the need for an additional or second neutralization step. In addition, the process of the invention is particularly useful for materials that strongly adhere to the hot dryer surfaces such as sodium dicambate in that the use of a mixer and dryer in combination produces carboxylic acid salts having a consistent pH. Furthermore, unlike prior art methods that use carboxylic acids dissolved in water, the molten carboxylic acid that is used in the method of the invention is essentially free of water thus reducing the amount of water that must be removed in drying. This in turn reduces the cost of drying the carboxylic acid salts. The resulting dry, carboxylic acid salts are at a desired pH (when dissolved in water) and can be immediately packaged and transported without having to blend the salts or buffer the salts to provide the desired pH.
These and other features and advantages of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description and accompanying drawing, which describe both the preferred and alternative embodiments of the present invention.